1. Field of the Invention
The present invention relates to new dimerization catalysts suitable for catalyzing reactions of organic isocyanates by heterogeneous catalysis, in particular as fixed bed catalysts, to a process for their preparation and to their use for the production of polyisocyanates containing uretdione and isocyanurate groups.
2. Description of the Prior Art
Catalysts for the dimerization of organic isocyanates are known (Alfred Hassner, Small Ring Heterocycles--Part 2, pages 522-526, John Wiley and Sons, New York, 1983). According to the teachings of DE-OS 34 32 081 (U.S. Pat. No. 4,614,785), tertiary phosphines are advantageously used.
These catalysts are generally used in a homogeneous phase, are often added in considerable quantities to the diisocyanates used as starting materials and must subsequently be inactivated.
The known processes for the preparation of dimerized diisocyanates (see e.g. DE-OS 16 70 720) are not optimally suitable for large scale commercial production. The main disadvantage of these processes is the need to use relatively large quantities of catalysts which require a correspondingly large quantity of inactivators such that the dimerized diisocyanates contain a relatively large proportion of unwanted foreign substances which have a deleterious effect on the properties of the polyurethanes produced from the diisocyanates.
According to the teachings of DE-OS 3 809 261, the quantity of catalyst may be reduced if the diisocyanate starting materials, e.g., hexamethylene diisocyanate (HDI) are free from carbon dioxide. Even when using the elaborate methods of purification, considerable quantities of homogeneous catalysts are still required which again necessitates correspondingly large quantities of inactivators.
It was therefore an object of the present invention to provide new dimerization catalysts for organic isocyanates which would be suitable for heterogeneous catalysis, in particular as fixed bed catalysts, so that the polyisocyanates having a uretdione and isocyanurate structure prepared from these catalysts can be worked up without adding the aforesaid inactivators after removal of the heterogeneous catalyst, preferably with the addition of small quantities of acid compounds as stabilizers.
This object has been achieved by providing the dimerization catalysts according to the invention described below.